[!123] Use files in CMake <-> Python interface instead of stdout

Merge branch 'feature/avoid-stdout-in-python-cmake-interface' into 'master'

ref:quality/dune-testtools stdout may always be cluttered with warnings etc.
and is not a robust tool in interfacing information into CMake.

This fixes [#134]

See merge request [!123]

  [#134]: gitlab.dune-project.org/NoneNone/issues/134
  [!123]: gitlab.dune-project.org/quality/dune-testtools/merge_requests/123


Closes #134